A chromium-free surface treatment liquid for zinc-iron alloy plated steel sheets and a method for producing the same

Abstract

The present application relates to the field of zinc-iron alloy plated steel sheet surface treatment, in particular to a chromium-free surface treatment liquid for zinc-iron alloy plated steel sheet and a preparation method thereof. The treatment liquid is prepared from the following raw materials: cationic water-based polyurethane emulsion, phosphoric acid or phosphate, silicate, titanium salt, silane coupling agent, and water. In the present application, triethanolamine is used as the catalyst and hydrophilic chain extender of cationic water-based polyurethane, and boric acid is used as the neutralizing agent. The obtained cationic water-based polyurethane structure contains ammonium borate salt, and the polyurethane emulsion has excellent stability without stratification and precipitation, and also has good corrosion protection effect on iron.

Description

Technical Field

[0001] This invention relates to the field of surface treatment of zinc-iron alloy coated steel sheets, specifically to a chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets and its preparation method. Background Technology

[0002] Zinc-iron alloy coated steel sheets are excellent metallic materials with good corrosion resistance, high strength and toughness, and good processing performance, making them widely used in construction, automotive, electronics, and home appliances. In the zinc-iron alloy coating on the steel sheet surface, zinc is the main component, with an iron content ranging from 7% to 30%. In humid air, the zinc-iron alloy coating is prone to corrosion, producing white rust or even red rust, affecting the appearance and paintability of the product. Therefore, surface passivation treatment is necessary for zinc-iron alloy coated steel sheets. Traditional zinc-iron alloy coated steel sheets are typically surface-treated using chromate solutions during manufacturing. However, chromate is toxic and poses potential hazards to the environment and human health. Currently, commercially available chromium-free passivation solutions for galvanized sheets exhibit good corrosion resistance for pure galvanized sheets. However, when used on zinc-iron alloy coated steel sheets, the zinc component on the coating surface is passivated, but the iron component remains unpassivated. This results in the exposed iron component not being effectively protected, leading to poor corrosion resistance in the passivated sheet. Therefore, there is a need to develop chromium-free passivating agents specifically designed for zinc-iron alloy coated steel sheets. Summary of the Invention

[0003] The purpose of this invention is to provide a chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets with excellent corrosion resistance, stamping processability, and environmental friendliness, as well as its preparation method.

[0004] The technical solution of the present invention is as follows:

[0005] A chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets is prepared from the following raw materials in parts by weight: 30-80 parts by weight of cationic aqueous polyurethane emulsion, 0.1-2 parts by weight of phosphoric acid or phosphate, 0.1-1 parts by weight of silicate, 0.1-2 parts by weight of titanium salt, 1-5 parts by weight of silane coupling agent, and water as the balance. The total parts by weight of all the above raw materials is 100.

[0006] Preferably, the chromium-free surface treatment solution for the zinc-iron alloy coated steel sheet is prepared from the following raw materials in parts by weight: 30-78 parts by weight of cationic aqueous polyurethane emulsion, 0.2-1.8 parts by weight of phosphate, 0.2-1 parts by weight of silicate, 0.2-1.8 parts by weight of titanium salt, 1-4 parts by weight of silane coupling agent, and 15-68.2 parts by weight of water;

[0007] Preferably, the phosphate is one or more of phosphoric acid, zinc phosphate, zinc dihydrogen phosphate, and ammonium dihydrogen phosphate.

[0008] Preferably, the silicate is one or more of potassium silicate, sodium silicate, and lithium silicate;

[0009] Preferably, the titanium salt is one or more of titanium oxysulfate, titanium oxalate, and fluorotitanic acid;

[0010] Preferably, the silane coupling agent is one or more of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltriethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane, more preferably γ-(2,3-epoxypropoxy)propyltrimethoxysilane and methyltriethoxysilane in a mass ratio of 1:1 or methyltriethoxysilane and γ-(2,3-epoxypropoxy)propyltriethoxysilane in a mass ratio of 1:1;

[0011] Preferably, the cationic aqueous polyurethane emulsion is prepared by the following method:

[0012] Step 1: Add 100 parts by weight of oligomeric diol, 50-100 parts by weight of diisocyanate, 2-8 parts by weight of triethanolamine and 10-20 parts by weight of organic solvent to the reaction vessel, heat to 50-90℃, stir and react for 1-3 hours, and then start cooling.

[0013] Step 2: When the temperature of the liquid in the reaction vessel drops to 30-40℃, add 5-15 parts by weight of boric acid aqueous solution and continue stirring for 20-40 minutes.

[0014] Step 3: Add 400-600 parts by weight of deionized water to the reaction vessel, stir and emulsify at a speed of 500-1000 rpm for 20-30 minutes to obtain a cationic aqueous polyurethane emulsion.

[0015] Preferably, the oligomeric diol is one or more of the following: polyether diol with a molecular weight of 600-1000, polyester diol with a molecular weight of 600-1000, and polytetrafluoroethylene diol with a molecular weight of 600-1000.

[0016] Preferably, the diisocyanate is isophorone diisocyanate and / or hexamethylene diisocyanate;

[0017] Preferably, the organic solvent is one or more of acetone, butanone, and N-methylpyrrolidone;

[0018] Preferably, the boric acid aqueous solution has a mass concentration of 5-30%;

[0019] This invention also discloses a method for preparing the above-mentioned chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets, the specific steps of which are as follows:

[0020] Phosphoric acid or phosphates, silicates, and titanium salts are added to deionized water and stirred to dissolve. The dissolved mixture is then added to a cationic aqueous polyurethane emulsion and stirred until homogeneous (preferably for 10-20 minutes). A silane coupling agent is then added to the mixture and stirred until homogeneous (preferably for 10-30 minutes) to obtain a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0022] ① After the zinc-iron alloy coated steel sheet of the present invention is coated with a chromium-free surface treatment liquid, during the drying process, the salt formed by triethanolamine and boric acid in the cationic waterborne polyurethane complexes with the metal on the coating surface, and the protective film formed has a good anti-corrosion effect on iron. At the same time, the cationic waterborne polyurethane forms an organic film with a silane coupling agent, and the phosphate (or phosphoric acid), silicate and titanium salt react with the coating metal to form an inorganic film, which has a good anti-corrosion effect on zinc.

[0023] ② Ordinary triethanolamine borate solution has a certain anti-corrosion effect on iron-based materials, but it is prone to stratification and precipitation, resulting in turbidity and poor anti-corrosion performance. In this invention, triethanolamine is used as a catalyst and hydrophilic chain extender for cationic waterborne polyurethane, and boric acid is used as a neutralizing agent. The resulting cationic waterborne polyurethane structure contains ammonium borate salt. The polyurethane emulsion has excellent stability, without stratification and precipitation, and at the same time has a good anti-corrosion effect on iron.

[0024] In the prior art, triethanolamine is usually used as a chain extender in cationic aqueous polyurethane. However, this application proposes that triethanolamine is used as a catalyst while acting as a chain extender, and is also neutralized with boric acid, so that ammonium borate salt is bound to the molecular structure of cationic aqueous polyurethane. Moreover, the stability of ammonium borate salt bound to the molecular structure of cationic aqueous polyurethane is better than that of aqueous solution of triethanolamine borate ester.

[0025] A common phenomenon with triethanolamine borate is its slow hydrolysis in aqueous solution into alcohols and borate compounds, especially under acidic conditions, which accelerates the hydrolysis process. This leads to stratification and precipitation in some formulations containing triethanolamine borate after a period of time. However, ammonium borate, when incorporated into the molecular structure of waterborne polyurethane, imparts good hydrophilicity to the polyurethane. Furthermore, ammonium borate exhibits good storage and thermal stability, resulting in good stability of the prepared waterborne polyurethane in aqueous solution. During the drying process to form an organic film, the ammonium borate reacts and bonds with the metal surface, resulting in excellent density and corrosion resistance. Attached Figure Description

[0026] Figure 1From left to right, the images show the morphology of steel plates passivated with chromium-free surface treatment solutions from Examples 1-6 after 120 hours of salt spray resistance testing. As can be seen from the images, after 120 hours of neutral salt spray testing, the steel plate passivated with the chromium-free surface treatment solution in Example 5 showed severe surface corrosion, with a corrosion area significantly greater than 10%, indicating poor salt spray corrosion resistance. In contrast, the steel plates passivated with the chromium-free surface treatment solutions in Examples 1-4 and Example 6 showed no obvious surface corrosion after 120 hours of neutral salt spray testing, with a corrosion area less than 5%, indicating relatively good salt spray corrosion resistance.

[0027] Figure 2 From left to right, the images show the cationic aqueous polyurethane emulsions prepared in Examples 1-6 after being placed at room temperature (25°C) for 6 months. As can be seen from the images, all six groups of cationic aqueous polyurethane emulsions are semi-transparent bluish-green emulsions, without any layering or precipitation, indicating that the cationic aqueous polyurethane emulsions prepared using this invention have excellent stability. Detailed Implementation

[0028] The technical solutions of the present invention will be further described in detail below with reference to the embodiments. It should be noted that the embodiments described below are intended to help those skilled in the art understand the present invention, and do not limit it in any way.

[0029] In the following examples, each part by weight is 100g.

[0030] Example 1: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This treatment solution is prepared from the following raw materials: 50 parts by weight of cationic aqueous polyurethane emulsion, 0.2 parts by weight of zinc dihydrogen phosphate, 0.2 parts by weight of potassium silicate, 0.4 parts by weight of titanium oxysulfate, 1 part by weight of silane coupling agent, and 48.2 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0031] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0032] Step 1: Add 100 parts by mass of polycarbonate diol with a molecular weight of 1000, 50 parts by mass of isophorone diisocyanate, 4 parts by mass of triethanolamine and 15 parts by mass of butanone to a reaction vessel, heat to 60°C, stir and react at 60°C for 2 hours, and then start cooling.

[0033] Step 2: When the temperature of the liquid in the reaction vessel drops to 35°C, add 5 parts by mass of a 10% boric acid aqueous solution, and continue stirring the reaction at 35°C for 30 minutes.

[0034] Step 3: Then add 450 parts by weight of deionized water to the above reaction vessel, stir and emulsify at room temperature (25°C, the same below) at a stirring speed of 500 rpm for 30 minutes to obtain cationic waterborne polyurethane emulsion.

[0035] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0036] At room temperature, 0.2 parts by mass of zinc dihydrogen phosphate, 0.2 parts by mass of potassium silicate, and 0.4 parts by mass of titanium oxysulfate are added to 48.2 parts by mass of deionized water and stirred to dissolve. The dissolved mixture is then added to 50 parts by mass of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 15 minutes, 0.5 parts by mass of γ-(2,3-epoxypropoxy)propyltrimethoxysilane and 0.5 parts by mass of methyltriethoxysilane, silane coupling agents, are added. Stirring is continued for 15 minutes until the mixture is homogeneous, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0037] Example 2: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This solution is prepared from the following raw materials: 60 parts by weight of cationic aqueous polyurethane emulsion, 0.8 parts by weight of zinc dihydrogen phosphate, 0.6 parts by weight of potassium silicate, 0.8 parts by weight of titanium oxysulfate, 2 parts by weight of silane coupling agent, and 35.8 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0038] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0039] 100 parts by weight of polypropylene glycol with a molecular weight of 800, 55 parts by weight of isophorone diisocyanate, 3 parts by weight of triethanolamine and 20 parts by weight of acetone were added to a reaction vessel, heated to 80°C, and stirred at 80°C for 1.5 hours before cooling down.

[0040] Step 2: When the temperature of the liquid in the reaction vessel drops to 40°C, add 5 parts by mass of an 8% boric acid aqueous solution, and continue stirring the reaction for 20 minutes while maintaining the temperature at 40°C.

[0041] Step 3: Then add 400 parts by weight of deionized water to the above reaction vessel, stir and emulsify at room temperature at a stirring speed of 900 rpm for 20 minutes to obtain cationic waterborne polyurethane emulsion.

[0042] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0043] At room temperature, 0.8 parts by mass of zinc dihydrogen phosphate, 0.6 parts by mass of potassium silicate, and 0.8 parts by mass of titanium oxysulfate are added to 35.8 parts by mass of deionized water and stirred to dissolve. The dissolved mixture is then added to 60 parts by mass of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 15 minutes, 2 parts by mass of silane coupling agent γ-(2,3-epoxypropoxy)propyltriethoxysilane are added, and stirring is continued for 15 minutes until the mixture is homogeneous, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0044] Example 3: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This solution is prepared from the following raw materials: 55 parts by weight of cationic aqueous polyurethane emulsion, 1.5 parts by weight of zinc dihydrogen phosphate, 0.8 parts by weight of potassium silicate, 1.8 parts by weight of titanium oxysulfate, 3 parts by weight of silane coupling agent, and 37.9 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0045] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0046] 100 parts by weight of polytetrahydrofuran ether diol with a molecular weight of 1000, 60 parts by weight of hexamethylene diisocyanate, 5 parts by weight of triethanolamine and 20 parts by weight of butanone were added to a reaction vessel, heated to 50°C, stirred at 50°C for 3 hours, and then cooled down.

[0047] Step 2: When the temperature of the liquid in the reaction vessel drops to 30°C, add 10 parts by mass of a 10% boric acid aqueous solution, and continue stirring the reaction for 30 minutes while maintaining the temperature at 30°C.

[0048] Step 3: Then add 420 parts by mass of deionized water to the above reaction vessel, stir and emulsify at room temperature at a stirring speed of 600 rpm for 30 minutes to obtain cationic waterborne polyurethane emulsion.

[0049] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0050] At room temperature, 1.5 parts by weight of zinc dihydrogen phosphate, 0.8 parts by weight of potassium silicate, and 1.8 parts by weight of titanium oxysulfate were added to 37.9 parts by weight of deionized water and stirred to dissolve. The dissolved mixture was then added to 55 parts by weight of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 20 minutes, 3 parts by weight of silane coupling agent γ-(2,3-epoxypropoxy)propyltriethoxysilane were added, and stirring was continued for 20 minutes until the mixture was homogeneous, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0051] Example 4: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This solution is prepared from the following raw materials: 78 parts by weight of cationic aqueous polyurethane emulsion, 1.8 parts by weight of zinc dihydrogen phosphate, 1 part by weight of potassium silicate, 0.2 parts by weight of titanium oxysulfate, 4 parts by weight of silane coupling agent, and 15 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0052] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0053] 100 parts by weight of polytetrahydrofuran ether diol with a molecular weight of 600, 95 parts by weight of isophorone diisocyanate, 8 parts by weight of triethanolamine and 20 parts by weight of N-methylpyrrolidone were added to a reaction vessel, the temperature was raised to 85°C, and the reaction was stirred at 85°C for 1.5 hours, and then the temperature was lowered.

[0054] Step 2: When the temperature of the liquid in the reaction vessel drops to 35°C, add 10 parts by mass of a 30% boric acid aqueous solution, and continue stirring the reaction at 35°C for 20 minutes.

[0055] Step 3: Continue to add 550 parts by mass of deionized water to the above reaction vessel, stir and emulsify at room temperature at a stirring speed of 800 rpm for 25 minutes to obtain cationic waterborne polyurethane emulsion.

[0056] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0057] At room temperature, 1.8 parts by mass of zinc dihydrogen phosphate, 1 part by mass of potassium silicate, and 0.2 parts by mass of titanium oxysulfate are added to 15 parts by mass of deionized water and stirred to dissolve. The dissolved mixture is then added to 78 parts by mass of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 20 minutes, 2 parts by mass of methyltriethoxysilane and 2 parts by mass of γ-(2,3-epoxypropoxy)propyltriethoxysilane are added to the mixture. Stirring is continued for 20 minutes until the mixture is homogeneous, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0058] Example 5: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This solution is prepared from the following raw materials: 30 parts by weight of cationic aqueous polyurethane emulsion, 0.2 parts by weight of zinc dihydrogen phosphate, 0.2 parts by weight of potassium silicate, 0.4 parts by weight of titanium oxysulfate, 1 part by weight of silane coupling agent, and 68.2 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0059] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0060] 100 parts by mass of polycarbonate diol with a molecular weight of 1000, 80 parts by mass of isophorone diisocyanate, 4 parts by mass of triethanolamine and 15 parts by mass of butanone were added to a reaction vessel, heated to 60°C, stirred at 60°C for 2 hours, and then cooled down.

[0061] Step 2: When the temperature of the liquid in the reaction vessel drops to 35°C, add 5 parts by mass of a 10% boric acid aqueous solution, and continue stirring the reaction at 35°C for 30 minutes.

[0062] Step 3: Continue to add 450 parts by mass of deionized water to the above reaction vessel, stir and emulsify at room temperature at a stirring speed of 1000 rpm for 20 minutes to obtain cationic waterborne polyurethane emulsion.

[0063] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0064] At room temperature, 0.2 parts by mass of zinc dihydrogen phosphate, 0.2 parts by mass of potassium silicate, and 0.4 parts by mass of titanium oxysulfate are added to 68.2 parts by mass of deionized water and stirred to dissolve. Then, the dissolved mixture is added to 30 parts by mass of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 15 minutes, 1 part by mass of silane coupling agent methyltriethoxysilane is added and stirring is continued for 15 minutes until the mixture is uniform, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0065] Example 6: A chromium-free surface treatment solution for zinc-iron alloy coated steel sheets. This solution is prepared from the following raw materials: 60 parts by weight of cationic aqueous polyurethane emulsion, 0.8 parts by weight of zinc dihydrogen phosphate, 0.6 parts by weight of potassium silicate, 0.8 parts by weight of titanium oxysulfate, 0.4 parts by weight of silane coupling agent, and 37.4 parts by weight of deionized water. Specifically, it is prepared by the following steps:

[0066] Step 1: The preparation method of cationic aqueous polyurethane emulsion is as follows:

[0067] 100 parts by mass of polypropylene glycol with a molecular weight of 800, 80 parts by mass of isophorone diisocyanate, 3 parts by mass of triethanolamine and 20 parts by mass of acetone were added to a reaction vessel, the temperature was raised to 80°C, and the reaction was stirred at 80°C for 1.5 hours, and then the temperature was lowered.

[0068] Step 2: When the temperature of the liquid in the reaction vessel drops to 40°C, add 5 parts by mass of an 8% boric acid aqueous solution, and continue stirring the reaction for 20 minutes while maintaining the temperature at 40°C.

[0069] Step 3: Continue to add 400 parts by mass of deionized water to the above reaction vessel, stir and emulsify at room temperature at a stirring speed of 800 rpm for 25 minutes to obtain cationic waterborne polyurethane emulsion.

[0070] Step 2: Preparation of chromium-free surface treatment solution for zinc-iron alloy coated steel sheets:

[0071] At room temperature, 0.8 parts by weight of zinc dihydrogen phosphate, 0.6 parts by weight of potassium silicate, and 0.8 parts by weight of titanium oxysulfate were added to 37.4 parts by weight of deionized water and stirred to dissolve. The dissolved mixture was then added to 60 parts by weight of the cationic aqueous polyurethane emulsion obtained in the first step. After stirring for 15 minutes, 0.2 parts by weight of silane coupling agent methyltriethoxysilane and 0.2 parts by weight of γ-(2,3-epoxypropoxy)propyltriethoxysilane were added. Stirring was continued for 15 minutes until the mixture was homogeneous, thus obtaining a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

[0072] Figure 2 From left to right, the images show the cationic aqueous polyurethane emulsions prepared in Examples 1-6 after being placed at room temperature (25°C) for 6 months. As can be seen from the images, all six groups of cationic aqueous polyurethane emulsions are semi-transparent bluish-green emulsions, without any layering or precipitation, indicating that the cationic aqueous polyurethane emulsions prepared using this invention have excellent stability.

[0073] Tests on the treatment effect of this invention on zinc-iron alloy coated steel plates:

[0074] The testing method is as follows:

[0075] Zinc-iron alloy coated steel sheets were coated with chromium-free surface treatment solution using the zinc-iron alloy coated steel sheets prepared in Examples 1-6. After drying at 90℃ to form a film, the film thickness was controlled at 1000-1200 mg / m², and the sheets were left to stand for 24 hours to obtain passivated zinc-iron alloy coated steel sheet samples. The zinc-iron alloy coated steel sheets used were provided by Benxi Iron & Steel Co., Ltd. The performance of each treated sample was tested according to the following method:

[0076] Salt spray resistance test: 120-hour spray test (conducted according to ASTM B117-16 standard): the corrosion area of ​​each treated sample should be <5%;

[0077] Processability test: The sample was subjected to an 0T bending test (in accordance with GB / T 30791-2014 standard). Transparent tape was used to stick and pull the bending area, and the passivation film in the bending area was observed to see if it fell off.

[0078] The test results of the zinc-iron alloy coated steel plates after treatment in Examples 1-6 are shown in Table 1 below:

[0079] Table 1

[0080]

[0081] As can be seen from Examples 1-4 above, after surface treatment of zinc-iron alloy coated steel sheets using the chromium-free surface treatment liquid of the present invention, the passivated plates exhibit good corrosion resistance and processing performance. Compared with Examples 1 and 5, the passivated plate in Example 5 has relatively poor corrosion resistance, mainly due to the insufficient amount of cationic waterborne polyurethane emulsion used in Example 5. Compared with Examples 2 and 6, the passivated plate in Example 6 experiences film peeling during bending and has poor processing performance, mainly because the amount of silane coupling agent used in Example 6 is too small.

Claims

1. A chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets, the liquid being prepared from the following raw materials in parts by weight: 30-80 parts by weight of cationic aqueous polyurethane emulsion, 0.1-2 parts by weight of phosphoric acid or phosphate, 0.1-1 parts by weight of silicate, 0.1-2 parts by weight of titanium salt, 1-5 parts by weight of silane coupling agent, and water as the balance, wherein the total parts by weight of all the above raw materials is 100. The cationic aqueous polyurethane emulsion was prepared by the following method: Step 1: Add 100 parts by weight of oligomeric diol, 50-100 parts by weight of diisocyanate, 2-8 parts by weight of triethanolamine and 10-20 parts by weight of organic solvent to the reaction vessel, heat to 50-90℃, stir and react for 1-3 hours, and then start cooling. Step 2: When the temperature of the liquid in the reaction vessel drops to 30-40℃, add 5-15 parts by weight of boric acid aqueous solution and continue stirring for 20-40 minutes. Step 3: Add 400-600 parts by weight of deionized water to the reaction vessel, stir and emulsify at a speed of 500-1000 rpm for 20-30 minutes to obtain a cationic waterborne polyurethane emulsion.

2. The chromium-free surface treatment liquid for zinc-iron alloy coated steel sheets according to claim 1, characterized in that, The phosphate is composed of one or more of zinc phosphate, zinc dihydrogen phosphate, and ammonium dihydrogen phosphate; the silicate is composed of one or more of potassium silicate, sodium silicate, and lithium silicate; and the titanium salt is composed of one or more of titanium oxysulfate, titanium oxalate, and fluorotitanic acid.

3. The chromium-free surface treatment liquid for zinc-iron alloy coated steel sheet according to claim 1, wherein the silane coupling agent is one or more of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltriethoxysilane, methyltrimethoxysilane, and methyltriethoxysilane.

4. The chromium-free surface treatment liquid for zinc-iron alloy coated steel sheet according to claim 1, wherein the oligomeric diol is one or more of the following: polyether diol with a molecular weight of 600-1000, polyester diol with a molecular weight of 600-1000, and polytetrafluoroethylene diol with a molecular weight of 600-1000; the diisocyanate is isophorone diisocyanate and / or hexamethylene diisocyanate; the organic solvent is one or more of the following: acetone, butanone, and N-methylpyrrolidone; and the boric acid aqueous solution has a mass concentration of 5-30%.

5. A method for preparing a chromium-free surface treatment liquid for zinc-iron alloy coated steel sheet according to any one of claims 1-4, comprising the following specific steps: Phosphoric acid or phosphates, silicates, and titanium salts are added to deionized water and stirred to dissolve. The dissolved mixture is then added to a cationic aqueous polyurethane emulsion and stirred until homogeneous. A silane coupling agent is added to the mixture and stirred until homogeneous to obtain a chromium-free surface treatment solution for zinc-iron alloy coated steel sheets.

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